Scientists in two nations are on the lookout for a rapidly spreading epidemic. This epidemic is under water, and it’s affecting starfish.

In September, divers in Vancouver Harbour and Howe Sound near Vancouver, British Columbia, started noticing the pizza-sized starfish known as sunflower stars wasting away and dying in large numbers. Marliave: “The sick ones tend to just fall apart in front of your eyes,” said Vancouver Aquarium biologist Jeff Marliave.

Marliave: “An arm will actually break off and crawl away. They turn into goo.”

Sunflower stars are not your typical starfish: they can have up to two dozen arms, spread three feet from tip to tip and weigh 12 pounds. They usually live deeper than other sea stars, so you’re unlikely to see these purple and orange giants unless you’re diving or pulling up a crab pot from the depths. Sunflower stars have become so abundant around Vancouver in the past decade—with some habitats becoming solid walls of sunflower stars—that Marliave said he actually welcomes the mystery disease knocking back their numbers.

Marliave: “I have been puzzling for upwards of a decade, how is this really capable killer and eater of sea life going to come under control?” he said. “This seems to be the solution. If you have too many of them for too long, they’re going to get sick.”

Sunflower stars are voracious predators. They can chew through almost anything in their path—from clams to kelp beds. A big question is will the fast-moving epidemic spread to other sea life and to other parts of the West Coast?

“It’s pretty hard dealing with epidemics in the ocean when they’re very much out sight, out of mind. For all we know, a lot have already died and are gone,” said marine ecologist Drew Harvell of Cornell University. The disease specialist is spending a year at the University of Washington’s Friday Harbor Laboratories on San Juan Island. Harvell said Olympic National Park biologists studying the more-familiar ‘ochre stars’– the brightly colored, stout-armed stars that dot rocky shorelines from Alaska to California – report seeing about 10 percent of them with a similar infection. In October, divers with the SeaDoc Society have reported small numbers of sunflower stars and three other species of sea stars wasting away in the San Juan Islands.

“Every population has sick animals,” said SeaDoc Society wildlife veterinarian Joe Gaydos, on a boat off Orcas Island between research dives.

“Are we just seeing sick animals because we’re looking for it, or is it an early sign of a large epidemic that may come through and wipe out a lot of animals?” Gaydos said.

Scientists in Washington and British Columbia are gathering sea stars. The healthy and diseased specimens are being sent off to wildlife laboratories to find out if the wasting disease is a virus, bacteria or something else entirely. Undersea life is often plagued by disease outbreaks, according to University of Washington marine ecologist Robert Paine, even though their causes are seldom identified.

“When these plagues have occurred in the past, identifying an unknown microorganism—to say, ‘this is what did it’— this is hard work. It’s difficult science,” Paine said. “We know there are a gazillion kinds of bacteria and viruses in the ocean.”